The present invention pertains to devices for altering the shot pattern obtainable from shotguns. In particular, the invention pertains to adjustable chokes for providing variable shot pattern with a single shotgun. This function has been addressed by a great variety of prior devices. Typically, one or more vents, barrel wall constrictions or enlargements and other structures are provided at the shotgun muzzle to compress or expand shot passing through the barrel. While such prior devices are somewhat successful in controlling shot pattern, they typically result in a firearm which is difficult to use or which detracts from the performance of the shotgun.
In both competition shooting and hunting, the additional weight of a shot pattern altering xe2x80x9cchokexe2x80x9d structure at the end of the gun barrel can render a shotgun more difficult to move and aim. In addition, in order to accomplish the desired functions, prior variable choke devices typically have radial dimensions significantly greater than the associated shotgun barrel. Because these larger structures are at the end of the gun barrel, the result is an obstruction to the user""s sight when aiming at a target. In addition, prior variable chokes require too much attention by the user to be successfully used in a hunting environment, where the user may have to quickly make adjustments to shot pattern depending upon rapidly changing events. These adjustments may also have to be accomplished in low light. Prior devices which require careful visual scrutiny of adjustment markings are likely to be improperly used in such conditions. The prior device disclosed in U.S. Pat. No. 2,629,958 to W. F. Roper et al. provides examples of these failings. The Roper device is relatively large: projecting outward from the outer diameter of the gun barrel to which it is mounted. The Roper device also provides adjustment indicia in the form of surface lettering which is likely to be difficult to use in low light conditions. Another example of this type of design is presented in U.S. Pat. No. 2,634,537 to R. V. Velez et al. What is needed is a low profile, low weight adjustable shotgun choke which does not interfere with the effective use of the connected shotgun and may be quickly adjusted without the need for careful scrutiny.
The present invention is a light weight low profile bore choke that is easily manually adjusted by the user. To minimize the choke profile, the choke is connected to the shotgun through a threaded connection within the muzzle. External threads on a choke inner sleeve are configured to mate with standard threads found in existing prior shotgun barrel muzzles or with custom threads specifically formed. By using a threaded connection internal to the gun barrel, the overall radial dimensions of the choke are minimized thereby reducing the profile visible to the user.
The choke is adjusted by the user by manually rotating an outer adjustment sleeve with respect to an inner bore sleeve which is fixed to the shotgun barrel. The adjustment sleeve has longitudinal grooves to increase grip. The adjustment sleeve has internal threads which mate with external threads on the bore sleeve. When the adjustment sleeve is rotated, a tapered adjustment surface within the adjustment sleeve forces elongated fingers on the bore sleeve radially inward to form a bore constriction. This constriction converges in the direction of the mouth of the choke to cause the choking effect. Various taper designs including those known in the prior art are applicable to the present invention. The adjustment sleeve rotates between two physical stops to provide the full range of adjustment. Indicia are also provided to provide visual indication of the choke condition. These are preferably circumferential grooves that are easily visible from any position surrounding the choke. One physical stop is formed by a spring clip that is located between the bore sleeve and the adjustment sleeve. It is secured rotationally to the bore sleeve and biased against the inside of the adjustment sleeve. As the adjustment sleeve is rotated, a clip end drags against the adjustment sleeve until aligning with an aperture in the adjustment sleeve. The adjustment sleeve is prevented from being moved past an open condition by the interference of the clip end in the aperture. This operation of the spring clip increases safety of the choke in that the adjustment sleeve cannot be accidentally loosened or removed from the bore sleeve. To reduce weight, the adjustment sleeve is preferably formed of titanium. Additional advantages of the invention will become obvious from the following details and figures.